Added Contribution clarification

Author: gvenzl

CONTRIBUTING.md

@@ -0,0 +1,25 @@
+# Contributing to oracle-db-examples
+
+*Copyright (c) 2017, Oracle and/or its affiliates. All rights reserved.*
+
+Pull requests can be made under
+[The Oracle Contributor Agreement](https://www.oracle.com/technetwork/community/oca-486395.html)
+(OCA).
+
+For pull requests to be accepted into oracle-db-examples, the bottom of
+your commit message must have the following line using your name and
+e-mail address as it appears in the OCA Signatories list.
+
+```
+Signed-off-by: Your Name <you@example.org>
+```
+
+This can be automatically added to pull requests by committing with:
+
+```
+git commit --signoff
+````
+
+Only pull requests from committers that can be verified as having
+signed the OCA can be accepted.
+